The present invention is related to an ellipsometer and alignment method for the incident angle of the ellipsometer, and particularly to an ellipsometer which is capable of performing three steps and one corrective sub step and precisely and repeatedly measuring surface characteristic of a specimen by scanning the surface by varying the incident angles of the light to the specimen and detecting the reflecting light from the specimen, and precision auto alignment method for incident angle of the ellipsometer.
It is well known that the ellipsometric method is one of the most accurate optical methods to study reflecting surfaces through the measurement of the optical constants of a material or thin-layer parameters.
However, it makes use of optical and mechanical components that are always prone to induce more or less important errors. In addition, the alignment relative to the incident and reflected light beams, always delicate, must not be altered by the rotation of some of the optical components. If this alignment is not accurate, a systematic error could be included in the ellipsometric measurements. Therefore, the incident angle of the light toward the specimen must be correctly aligned. Here, the process to correct the altered incident angle is called as xe2x80x9cincident angle alignmentxe2x80x9d, which comes from misalignments of components including the specimen.
Recently, some investigators have shown that many cases of angle-of-incidence dependence of optical constants of specula surfaces can be attributed to azimuthal misalignments.
FIG. 1 shows a schematic block diagram of a conventional ellipsometer. As shown in the drawing, the conventional ellipsometer comprises a polarizing unit 1 for inputting a light from a light source and polarizing the light, a specimen stage 4 for supporting a specimen thereon and letting the polarized light from the polarizing unit 1 be incident on the specimen, a detecting unit 3 for detecting the reflecting light from the specimen and analyzing the detected light, and a focusing microscopy 4 for adjusting the incident angle of the polarized light from the polarizing unit 1 to the specimen.
FIG. 2 shows a detailed view of FIG. 1. As shown in the Figure, the polarizing unit 1 includes a polarizer 1b polarizing the light from the light source and a modulator 1a modulating the polarized light from the polarizer 1b and outputting the modulated light to the specimen 4a. Also, the detecting unit 3 includes an analyzer 3a inputting the reflection light from the specimen 4a and analyzing its polarizing state, and a detector 3b changing the light from the analyzing light into an electrical signal. Here, the dashed line stands for an optical path when the components are in perfect alignment and the continuous lines denotes an optical path when they are in misalignment.
However, the incident angle is altered by the misalignments of the components as well as the specimen stage. Namely, according to the state of the specimen stage 4 and the position of the specimen 4a on the specimen stage 4, there are h translation error and xcex1 tilt angle error and xcex2 tilt angle error. The h translation error occurs when components such as the polarizing prism and the light source, are set and the xcex1 tilt angle error and xcex2 tilt angle error rise, ie, when a new specimen is put on the specimen stage. The xcex1 and xcex2 tilt angle errors and the h translation error of the specimen stage may arise because the dimension of the present specimen may differ from the previous.
When a specimen, for example SiO2 (of 100 nm) Si, is on the specimen stage 4 and the incident angle of the light onto the specimen 4a is 70, the xcex1 and xcex2 tilt angle errors and translation error occurs in the process for the measurements of the specimen and their quantities are as below.
When the analyzer prism is misaligned by 3 tilt angle error and rotates for measurement, the spot also rotates and is partially blocked by the detector""s entrance aperture. The trajectory of the spot when the analyzer prism rotates is shown in FIG. 3, which was embodied in a Cartesian coordinate system from the electrical signals converted from the detecting light. The light is totally received or partially received from the detector""s entrance aperture. Here, the dashed circle, G1, is a trajectory of the detector""s entrance aperture, G2 is the trajectory of the spots and G3 is a trajectory of moving the center of the focus according to the rotation of the detector.
Thus, the signal of detector changes according to the fraction of light that is arrived at detector through the entrance aperture, as shown in FIG. 4.
FIG. 4 shows graphs where the detector converts the reflected signal into an electrical signal when the analyzer prism is misaligned. Here, G4 is a trajectory drawn by the detected signal at the detector when the components are misaligned, G5 is a trajectory drawn by the detected signal when the components 25 are perfectly aligned, and G6 is a trajectory drawn by a fraction of light which has arrived at the detector through the entrance aperture. Here, a value of 1 for the fraction of light at the axis of the normalized signal magnitude indicates that the spot has totally arrived at detector through the entrance aperture, provided that the detector is not saturated. Thus, the signal of the detector is the product of the intensity and the fraction of light.
Meanwhile, the orientation angle error of analyzer is, 3 the film thickness is calculated as 95.31 m nm, that is a 4.79 nm thickness error. In this case, the alignment precision depends on the precision of manufacture and assemblage.
When exchanging a specimen, the tilt angle errors and translation error of the specimen stage may arise. Because the dimension of the currently putting specimen may differ from that of the previous one. The alpha tilt angle error does not alter the incident angle but both beta tilt angle error and translation error alter the incident angle. The beta tilt angle error and translation error are related by the geometry of system.
FIG. 5 shows a measurement error of the film thickness with tilt angle error of specimen stage, which is the same as the beta tilt angle error, and FIG. 6 shows a measurement error of the film thickness with translation error of specimen stage, which is geometrically related with translation error.
The measurement error was proportional to both xcex2 tilt angle error and h translation error. 1 xc3x85 measurement error of film thickness is due to each 0.022xc2x0 tilt angle error and 80 xcexcm translation error of the specimen stage. From these results and rule of thumb, the resolutions of the specimen stage should be higher than 0.0022xc2x0 and 8 xcexcm each for tilt and translation motion, for the purpose of assuring 1 xc3x85 precision of measurement.
However, the conventional ellipsometers must use auxiliary equipment such as a focusing microscopy and 3-axis specimen stage for alignment of incident angle, which makes the system expensive. Also, it is difficult to compensate for the misalignment of incident angle, thereby it may not accurately analyze the surface characteristic of a specimen and not provide the complete information obtained therefrom either.
Also, another conventional ellipsometer using step motors may easily align the spot at the center of the detector""s aperture though, but it still has a translation error of specimen stage cause incident angle error.
The main object of the present invention is to provide an ellipsometer which is capable of supplying a various incident angles of light onto a specimen and easily aligning the incident angles of light using kinematic coupling.
Another object of the present invention is to provide an ellipsometer which is capable of supplying a various incident angles of light onto a specimen and easily aligning the incident angles of light using a 3-axis specimen stage and a detector outputting a signal.
Still another object of the present invention is to provide a 3 step auto-alignment algorithm for supplying a various incident angles of light onto a specimen and easily aligning the incident angles of light using a 3 axis specimen stage and a detector outputting a signal.
In order to achieve the object of the present invention, there is provided an ellipsometer for aligning incident angle, wherein the ellipsometer comprises: a main frame shaping half circle and flat surface on which a plurality of grooves are radial and circumferential directionally carved; a specimen stage, which is installed at the groove carved surface of the main frame for tilting a specimen on an upper surface of the specimen stage with respect to horizontal direction and translating the specimen upward and downward; a polarizing unit, which is movably positioned on the groove-carved surface of the main frame for polarizing a light from a light source and outputting the polarized light on the specimen,; and a light detecting unit, which is movably positioned on the groove-carved surface to receive light reflected from the specimen.
In order to achieve the object of the present invention, a precision auto alignment method is provided to align the incident angle of an ellipsometer, wherein the precision auto alignment method comprises the steps of: measuring tilt and translating angle errors according to incident angles of a polarizing unit; compensating each error by moving a light spot reflecting from the specimen onto a center of the detector""s entrance aperture; calculating the tilt and translating angle errors by repeatedly performing the measuring and compensating steps above; and correctly aligning incident angle for the ellipsometer by adjusting the tilt and translating angles accordingly.